1. Field of the Invention
The present invention relates to a cooling device for an engine that can efficiently raise the temperature of cooling water within a short period of time during the warm-up operation of an automobile and makes it possible to simplify the structure of the cooling device.
2. Description of the Related Art
The demand for increased fuel efficiency and improved purification of exhaust gases in automobiles has been rapidly growing in recent years. Accordingly, means for rapidly raising the temperature of cooling water within the warm-up period immediately after the engine has been started and attaining the best possible state of the cooling water within a short period of time after the engine has been started have been actively researched, developed, and put to practical use with the object of increasing fuel efficiency and improving purification of exhaust gases in vehicles. Japanese Patent Application Laid-open No. 2007-120381 is a document aimed at such a rapid increase in cooling water temperature.
Japanese Patent Application Laid-open No. 2007-120381 discloses a configuration in which a radiator circulation flow channel having a radiator disposed therein, a heater circulation flow channel having a heater core disposed therein, and a bypass circulation flow channel where no heat exchange is performed are provided in parallel, a control valve is provided in the bypass circulation flow channel, and the control valve and a thermostat are disposed in series. The control valve is a pressure control valve and this valve opens when a pressure inside the bypass circulation flow channel becomes equal to or higher than a predetermined value. Within the warm-up period in which the cooling water temperature is low, the radiator circulation flow channel is shut down by the thermostat, the cooling water does not flow through the radiator, and the cooling water is prevented from cooling. Furthermore, within the warm-up period in which the cooling water temperature is low, the cooling water flows only through the heater circulation flow channel where the heater core is disposed, or the cooling water flows into the heater circulation flow channel and bypass circulation flow channel and the cooling water temperature is rapidly raised.
With Japanese Patent Application Laid-open No. 2007-120381, rapid increase in cooling water temperature can be attained. However, the following problems remain unresolved in the configuration of Japanese Patent Application Laid-open No. 2007-120381. First, when the water temperature is low before the control valve is opened (when the control valve is closed), both the thermostat and the control valve are almost closed. Therefore, almost the entire cooling water flows in the heater circulation flow channel where the heater core is disposed. Because the heater core is a heat dissipation source, heating of the cooling water is delayed due to heat dissipation from the heater core.
Furthermore, when the water temperature is low and when the pump operates at a high revolution speed such that the control valve is not open (a state in which the predetermined pressure is not reached), the cooling water does not flow in the bypass circulation flow channel and flows into the heater circulation flow channel where the heater core is disposed. As a result, a larger amount of cooling water flows into the heater core. For this reason, a large amount of heat of the cooling water is taken away by the heater core and, therefore, the cooling water temperature rises even more slowly. In addition, at a pressure or water temperature at which the thermostat is closed and the control valve is opened, the cooling water does not flow into the radiator, and because the control valve is opened, the cooling water flows into the heater circulation flow channel and bypass circulation flow channel.
However, the bypass circulation flow channel has a structure different from that of the heater circulation flow channel and no specific objects offering resistance to the passing water are present in the bypass circulation flow channel. For this reason, a major portion of cooling water flows into the bypass circulation flow channel and because no resistance is offered to the flow, the flow speed of the cooling water flowing in the bypass circulation flow channel become extremely high. An extremely high flow speed of cooling water means that the interval in which the cooling water stays in the engine, which is a heat generation source, is short, the amount of heat picked up by the cooling water is small, and temperature increase in cooling water is delayed. As described hereinabove, where the control valve is opened, most of cooling water flows in the bypass circulation flow channel that offers little resistance to the passing water, and the amount of cooling water flowing in the heater circulation flow channel decreases. Therefore, the capacity to warm up the heater core decreases. As a result, the warming temperature decreases.
Furthermore, because of very rapid opening and closing of the control valve, the flow rate (flow speed) of cooling water flowing in the bypass circulation flow channel rapidly changes and there is also a risk of pulsations or a water hammer phenomenon occurring inside a piping. Because pipes of a cooling water circuit for a vehicle are connected in a continuous manner along the entire perimeter, a shock wave generated in the piping propagates through the entire cooling water circuit. As a result, because the heater core is disposed in the cooling water circuit in a location closest to the driver to warm the inside of the vehicle, the shock wave can cause vibrations of the heater core and discomfort (noise or vibrations) to the driver.